1. Field of the Invention
The invention is generally related to the evaluation of subterranean formations, and more particularly to measuring standoff and borehole geometry.
2. Background of the Invention
Boreholes drilled in subterranean formations such as oilfields often have an irregular shape. In particular, the borehole wall is not perfectly smooth. The magnitude of irregularity may vary along the length of a given borehole, and be particularly great where the borehole traverses weak, highly stressed or fractured rock. Borehole shape (a.k.a., geometry) can provide an indication of the mechanical stability of the borehole, and can affect the reliability of some logging measurements. It is therefore useful to know borehole geometry.
It is known to use caliper measurements to evaluate the geometry of boreholes. On wireline tools, caliper measurements are local diameter measurements made either with mechanical arms or ultrasonic pulse/echoes. On logging-while-drilling tools, ultrasonic pulse/echoes are used. Caliper measurements can be combined to provide an indication of borehole geometry, i.e., two-dimensional or three-dimensional representations.